1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly, to a process control method in wafer grinding capable of processing a wafer thickness with high accuracy.
2. Description of the Background Art
In manufacturing a semiconductor device, higher density of a package is pursued in LSI by three-dimensional packaging, and a wafer becomes thinner such that a wafer thickness is approximately 25 μm at the completion of the process.
In power devices such as an insulated gate bipolar transistor (IGBT) and a MOS field-effect transistor (MOSFET), power semiconductor devices are widely used as inverter circuits for industrial motors or automobile motors, a power supply device of a large-capacity server, and a semiconductor switch for an uninterruptible power supply device or the like. In those power semiconductor devices, a semiconductor substrate is processed to be thin for improving conduction performance such as on-state characteristics.
Nowadays, for improvements in cost and characteristics, devices are manufactured from wafer materials manufactured by the floating zone (FZ) method, with the use of an extra-thin wafer process in which a wafer is made thinner to approximately 60 μm.
Typically, in processing a wafer thinner, there is used wet etching or dry etching for removing processing distortions generated in polishing by back grinding or polishing and mechanical polishing, and then an electrode is formed on a back surface side by diffusion layer formation using ion implantation or thermal treatment, sputtering or the like.
In that case, for example, a protective tape mainly formed of polyethylene terephthalate (PET) is conventionally used as a reinforcing member of a thinner wafer, and along with the wafer becoming thinner, the protective tape becomes thicker to approximately several hundreds of μm for ensuring the strength. Further, in recent years, there has been proposed the method of bonding a wafer to a support plate using a glass material and then performing the steps following mechanical polishing because the protective tape cannot withstand treatment, and a wafer warps or deflects considerably by reinforcement with a protective tape.
In using a protective member, the device performance is influenced by the wafer thickness after the wafer is processed to be smaller, and thus the protective member becomes thicker, whereas precise thickness control is required for wafers.
To cope with the problem in which the wafer thickness is required to be controlled with high accuracy as described above, Japanese Patent Application Laid-Open No. 2001-300847 proposes the technique of mounting optical measurement means onto a grinding machine to accurately measure a wafer thickness in grinding.
Further, Japanese Patent Application Laid-Open No. 2009-111238 proposes the method of measuring a thickness in which a grinding fluid or a grinding material scattered in wafer grounding is prevented from becoming an obstacle to thickness measurement by means of a cover that covers a cleaning nozzle and an optical system.
In the above-mentioned grinding methods, it is possible to control a wafer thickness at one point in a plane or on a circumference with a uniform radius, but control of the thickness distribution in a wafer is not considered. Accordingly, in a case where a conventional grinding method is used, there is a drawback that an average value of the thickness falls out of a desired value.
Further, also in a case where a tilt of a stage to which a wafer sticks fast or a tilt of a grinding stone for grinding a wafer falls out of desired value as the number of processing pieces increases, such tilt cannot be detected by a conventional method, leading to a problem that in-plane uniformity of wafer thickness becomes worse.